The present invention relates to a bicycle crank set and bottom bracket.
Most bottom brackets hold left and right bearings internally via a separate cup that is threaded into the bottom bracket shell itself. The separate cup either alone or in combination with the bottom bracket shell aligns the left and right bearings to a rotating axis of the crankarms. For example, one common type of bottom bracket is the Shimano-style cartridge bottom bracket. The cartridge bearings are inserted into the bottom bracket shell of the bicycle frame. Cups are threaded into opposed sides of the bottom bracket shell to hold the cartridge in place. There are two deficiencies in these types of bottom bracket shell assemblies. First, the threads of the bottom bracket shell increases the cost of manufacturing the bicycle frame. Second, the bearings must be sized to fit between the cups and the spindle rather than for bearing performance.
Another type of bottom bracket was recently introduced by Shimano wherein the left and right bearings are external to the bottom bracket shell. The left and right bearings are held in place with respect to the bottom bracket shell via a threaded cup threaded into the bottom bracket shell. Unfortunately, the threads formed in the bottom bracket shell increases the cost of manufacturing the bicycle frame and possibility of damage to bicycle during assembly.
To eliminate the threads in the bottom bracket shell, some manufacturers have press fit the left and right bearings internally into the left and right side portions of the bottom bracket shell. Unfortunately, these press fit systems are also susceptible to improper fit because the manufacturing tolerance required for press fits are difficult to achieve using standard bicycle frame building practices.
Accordingly, there is a need in the art for an improved bicycle bottom bracket.